*2.4. FTIR Spectroscopy*

Line (1) in Figure 4 represents the FTIR spectra of the as received TO. In the same figure, Line (2) is assigned to the as received natural zeolite FTIR spectra, and Line (3) to the modified rich in thymol natural zeolite TO@NZ. *Gels* **2022**, *8*, x FOR PEER REVIEW 6 of 24

**Figure 4.** FTIR plots of: (1) TO as received, (2) NZ as received, (3) modified TO@NZ hybrid **Figure 4.** FTIR plots of: (1) TO as received, (2) NZ as received, (3) modified TO@NZ hybrid nanostructure.

nanostructure. In the FTIR plot of the TO material, i.e., Line (1) in Figure 4, the bands at ~3400 cm <sup>−</sup><sup>1</sup> and at ~3500 cm−1 are assigned to hydrogen-bonded OH stretching, and the bands at ~3100–3000 cm−1 are devoted to aromatic and alkenic C-H=C-H stretch vibrations. Three more bands are observed between 2800 and 3000 cm−1 which are attributed to the C-H stretch vibration of aliphatic CH2 bonds. These bands are the strongest evidence that TO was adsorbed on NZ substrate because they are not covered by pristine NZ bands. For wavenumbers between 1500 cm−1 and 1300 cm−1 as well as for lower than 1000 cm−1, there are several bands assigned to TO which are attributed to the C-H bending of the aliphatic In the FTIR plot of the TO material, i.e., Line (1) in Figure 4, the bands at ~3400 cm−<sup>1</sup> and at ~3500 cm−<sup>1</sup> are assigned to hydrogen-bonded OH stretching, and the bands at ~3100–3000 cm−<sup>1</sup> are devoted to aromatic and alkenic C-H=C-H stretch vibrations. Three more bands are observed between 2800 and 3000 cm−<sup>1</sup> which are attributed to the C-H stretch vibration of aliphatic CH<sup>2</sup> bonds. These bands are the strongest evidence that TO was adsorbed on NZ substrate because they are not covered by pristine NZ bands. For wavenumbers between 1500 cm−<sup>1</sup> and 1300 cm−<sup>1</sup> as well as for lower than 1000 cm−1, there are several bands assigned to TO which are attributed to the C-H bending of the aliphatic CH<sup>2</sup> groups and C-O-H bending. These bands do not be overlapped with those of NZ. spectra and thus they could be visible in an NZ spectrum with adsorbed TO [15,38].

CH2 groups and C-O-H bending. These bands do not be overlapped with those of NZ. spectra and thus they could be visible in an NZ spectrum with adsorbed TO [15,38]. In the FTIR plots of NZ and TO@NZ powders ((see line (2) and line (3) in Figure 4), the bands at 3619 and 3436 cm−<sup>1</sup> are assigned to the OH group stretching mode. The band at 1650 cm−1 corresponds to the OH group bending mode. The band at 1090 cm−1 to the Si-O stretching vibration and at 468 cm−1 to the -SiO4- bending mode [39–41]. It is obvious from the FTIR plot of TO@NZ that characteristic bands of TO exist in the range of 2800– 3100 cm−1, 1300–1500 cm−1, and 500–1000 cm−1. This indicates that the adsorption of TO In the FTIR plots of NZ and TO@NZ powders ((see line (2) and line (3) in Figure 4), the bands at 3619 and 3436 cm−<sup>1</sup> are assigned to the OH group stretching mode. The band at 1650 cm−<sup>1</sup> corresponds to the OH group bending mode. The band at 1090 cm−<sup>1</sup> to the Si-O stretching vibration and at 468 cm−<sup>1</sup> to the -SiO4- bending mode [39–41]. It is obvious from the FTIR plot of TO@NZ that characteristic bands of TO exist in the range of 2800–3100 cm−<sup>1</sup> , 1300–1500 cm−<sup>1</sup> , and 500–1000 cm−<sup>1</sup> . This indicates that the adsorption of TO molecules in the NZ occurs. The absence of band shift between NZ and TO@NZ plots means that the adsorption process is rather a physisorption than chemisorption.

molecules in the NZ occurs. The absence of band shift between NZ and TO@NZ plots

Line (1) in Figure 5 depicts the FTIR plots of ALG/G while Line (2) shows the FTIR

means that the adsorption process is rather a physisorption than chemisorption.

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Line (1) in Figure 5 depicts the FTIR plots of ALG/G while Line (2) shows the FTIR spectra of ALG/G/NZ material, and Line (3) of ALG/G/TO@NZ films.

**Figure 5.** Representative FTIR spectra of (1) ALG/G, (2) ALG/G/NZ, (3) ALG/G/TO@NZ obtained films. **Figure 5.** Representative FTIR spectra of (1) ALG/G, (2) ALG/G/NZ, (3) ALG/G/TO@NZ obtained films.

Line 1 of Figure 5 corresponds to the FTIR plot of pure ALG/G film while line 2 is a representative FTIR plot of ALG/G/10NZ. Finally, line 3 is assigned to FTIR measurements of ALG/G/10TO@NZ nanocomposite films. The characteristic sodium-alginate peaks are observed in all plots. A broad band at 3.428 cm−1 is assigned to hydrogen-bonded O–H stretching vibrations [42]. The band at 1635 cm−1 is attributed to the asymmetric stretching vibration of COO groups, the band at 1419 cm−1 to the symmetric stretching vibration of COO groups, and the band at 1050 cm−1 to the elongation of C-O groups [43]. It is obvious from lines (2) and (3) that the addition of NZ and TO@NZ hybrid nanostructures causes an increase to the bands at 3.428 cm−1 and 1635 cm−1 which could be attributed to strong interactions of ALG chains with NZ and TO@NZ hybrid nanostructures. This interaction is higher in the case of the TO@NZ hybrid nanostructure. Thus, it is revealed that modified TO@NZ hybrid structure interacts better with ALG/G matrix compared to the relevant of the pure NZ material. Furthermore, the absence of TO peaks in ALG/G/10TO@NZ spectra indicates that the TO molecules are not in the surface but in the inner area of the ALG/G Line 1 of Figure 5 corresponds to the FTIR plot of pure ALG/G film while line 2 is a representative FTIR plot of ALG/G/10NZ. Finally, line 3 is assigned to FTIR measurements of ALG/G/10TO@NZ nanocomposite films. The characteristic sodium-alginate peaks are observed in all plots. A broad band at 3.428 cm−<sup>1</sup> is assigned to hydrogen-bonded O–H stretching vibrations [42]. The band at 1635 cm−<sup>1</sup> is attributed to the asymmetric stretching vibration of COO groups, the band at 1419 cm−<sup>1</sup> to the symmetric stretching vibration of COO groups, and the band at 1050 cm−<sup>1</sup> to the elongation of C-O groups [43]. It is obvious from lines (2) and (3) that the addition of NZ and TO@NZ hybrid nanostructures causes an increase to the bands at 3.428 cm−<sup>1</sup> and 1635 cm−<sup>1</sup> which could be attributed to strong interactions of ALG chains with NZ and TO@NZ hybrid nanostructures. This interaction is higher in the case of the TO@NZ hybrid nanostructure. Thus, it is revealed that modified TO@NZ hybrid structure interacts better with ALG/G matrix compared to the relevant of the pure NZ material. Furthermore, the absence of TO peaks in ALG/G/10TO@NZ spectra indicates that the TO molecules are not in the surface but in the inner area of the ALG/G matrix and supports the relaxation between the NZ material and the ALG/G matrix.

### matrix and supports the relaxation between the NZ material and the ALG/G matrix. *2.5. SEM Images*

*2.5. SEM Images*  A SEM instrument equipped with an EDS detector was used to investigate the surface/cross-section morphology of the pure ALG/G film as well as of the ALG/G/xNZ and ALG/G/xTO@NZ hybrid nanocomposite films. The results confirmed that the NZ and the TO@NZ hybrid nanostructures were homogeneously dispersed in the ALG/G polymeric matrix. The chemical elements contained in the pure and final nanocomposite active packaging films were identified by carrying out EDS analysis on the surface of the materials. A SEM instrument equipped with an EDS detector was used to investigate the surface/cross-section morphology of the pure ALG/G film as well as of the ALG/G/xNZ and ALG/G/xTO@NZ hybrid nanocomposite films. The results confirmed that the NZ and the TO@NZ hybrid nanostructures were homogeneously dispersed in the ALG/G polymeric matrix. The chemical elements contained in the pure and final nanocomposite active packaging films were identified by carrying out EDS analysis on the surface of the materials.

The SEM images in Figure 6a,b show the expected smooth morphology inside and outside of the neat ALG/G polymer matrix. The EDS spectra in Figure 6c certify the existence of carbon (C), oxygen (O), and sodium (Na) on the surface of such films which is expected because of the sodium alginate. Figures 7e, 8e and 9e show EDS chemical analysis of nanocomposite active packaging films with different concentrations of pure NZ and TO@NZ hybrid nanostructure i.e., 5, 10, and 15% wt. In addition to the above mentioned The SEM images in Figure 6a,b show the expected smooth morphology inside and outside of the neat ALG/G polymer matrix. The EDS spectra in Figure 6c certify the existence of carbon (C), oxygen (O), and sodium (Na) on the surface of such films which is expected because of the sodium alginate. Figures 7e, 8e and 9e show EDS chemical analysis of nanocomposite active packaging films with different concentrations of pure NZ and TO@NZ hybrid nanostructure i.e., 5, 10, and 15% wt. In addition to the above mentioned presence of (C), (O), and (Na),

presence of (C), (O), and (Na), the presence of typical elements, such as Si, Al, Fe, K, and Ca, confirm the existence of NZ and TO@NZ in such nanocomposite films. Moreover, the the presence of typical elements, such as Si, Al, Fe, K, and Ca, confirm the existence of NZ and TO@NZ in such nanocomposite films. Moreover, the increase of (Na) content of the ALG/G/xNZ films i.e., ~10% compared to the relevant content of the pure ALG/G films i.e., ~2% indicates the incorporation of the natural zeolite into the polymer matrix. Surface and relative cross-section images of ALG/G/xNZ and ALG/G/xTO@NZ with different ratios x of NZ and TO@NZ are presented in Figures 7–9. increase of (Na) content of the ALG/G/xNZ films i.e., ~10% compared to the relevant content of the pure ALG/G films i.e., ~2% indicates the incorporation of the natural zeolite into the polymer matrix. Surface and relative cross-section images of ALG/G/xNZ and ALG/G/xTO@NZ with different ratios x of NZ and TO@NZ are presented in Figures 7–9.

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**Figure 6.** (**a**) SEM images of the surface and (**b**) cross-section for the pure film of ALG/G. (**c**) Energy dispersive spectrometer (EDS) spectrum and relative elemental analysis of the surface (inset) from the SEM image (**a**). **Figure 6.** (**a**) SEM images of the surface and (**b**) cross-section for the pure film of ALG/G. (**c**) Energy dispersive spectrometer (EDS) spectrum and relative elemental analysis of the surface (inset) from the SEM image (**a**). *Gels* **2022**, *8*, x FOR PEER REVIEW 9 of 24

> **Figure 7.** (**a**,**c**) SEM images of the surface and (**b**,**d**) cross-section for the nanocomposite films of ALG/G/5NZ (**a**,**b**) and ALG/G/5TO@NZ (**c**,**d**) respectively. (**e**) Energy dispersive spectrometer (EDS)

spectrum and relative elemental analysis of the surface (inset) from the SEM image (**a**).

**Figure 7.** *Cont*.

**Figure 7.** (**a**,**c**) SEM images of the surface and (**b**,**d**) cross-section for the nanocomposite films of ALG/G/5NZ (**a**,**b**) and ALG/G/5TO@NZ (**c**,**d**) respectively. (**e**) Energy dispersive spectrometer (EDS) spectrum and relative elemental analysis of the surface (inset) from the SEM image (**a**). **Figure 7.** (**a**,**c**) SEM images of the surface and (**b**,**d**) cross-section for the nanocomposite films of ALG/G/5NZ (**a**,**b**) and ALG/G/5TO@NZ (**c**,**d**) respectively. (**e**) Energy dispersive spectrometer (EDS) spectrum and relative elemental analysis of the surface (inset) from the SEM image (**a**). *Gels* **2022**, *8*, x FOR PEER REVIEW 10 of 24

**Figure 8.** (**a**,**c**) SEM images of surface and (**b**,**d**) cross-section for the nanocomposite films of ALG/G/10NZ (**a**,**b**) and (**c**,**d**) ALG/G/10TO@NZ (**c**,**d**) respectively. (**e**) Energy dispersive spectrometer (EDS) spectrum and relative elemental analysis of the surface (inset) from the SEM image (**a**).

**Figure 8.** *Cont*.

**Figure 8.** (**a**,**c**) SEM images of surface and (**b**,**d**) cross-section for the nanocomposite films of ALG/G/10NZ (**a**,**b**) and (**c**,**d**) ALG/G/10TO@NZ (**c**,**d**) respectively. (**e**) Energy dispersive spectrometer (EDS) spectrum and relative elemental analysis of the surface (inset) from the SEM image (**a**). **Figure 8.** (**a**,**c**) SEM images of surface and (**b**,**d**) cross-section for the nanocomposite films of ALG/G/10NZ (**a**,**b**) and (**c**,**d**) ALG/G/10TO@NZ (**c**,**d**) respectively. (**e**) Energy dispersive spectrometer (EDS) spectrum and relative elemental analysis of the surface (inset) from the SEM image (**a**).

It is obvious from Figures 7–9 that after the incorporation into the polymer matrix, the increase of the content of NZ or TO@NZ nanocomposite material caused an increase to the aggregation degree. Nevertheless, SEM images of the final nanocomposite films show that the nanohybrids were homogeneously dispersed, which indicates their enhanced compatibility with the polymer matrix. Moreover, SEM surface and cross-section images were shown more homogenous dispersion in the case of TO@NZ hybrid nanostructure in nanocomposite films compared to the relevant of pure NZ. This means that the TO@NZ hybrid nanostructure was incorporated significantly better in the polymer matrix compared to the incorporation of the respective pure NZ. *Gels* **2022**, *8*, x FOR PEER REVIEW 11 of 24

**Figure 9.** *Cont*.

*2.6. Tensile Properties* 

Table 1.

mer matrix compared to the incorporation of the respective pure NZ.

**Figure 9.** (**a**,**c**) SEM images of surface and (**b**,**d**) cross-section for the nanocomposite films of ALG/G/15NZ (**a**,**c**) and ALG/G/15TO@NZ (**c**,**d**) respectively. (**e**) Energy dispersive spectrometer (EDS) spectrum and relative elemental analysis of the surface (inset) from the SEM image (**a**).

It is obvious from Figures 7–9 that after the incorporation into the polymer matrix, the increase of the content of NZ or TO@NZ nanocomposite material caused an increase to the aggregation degree. Nevertheless, SEM images of the final nanocomposite films show that the nanohybrids were homogeneously dispersed, which indicates their enhanced compatibility with the polymer matrix. Moreover, SEM surface and cross-section images were shown more homogenous dispersion in the case of TO@NZ hybrid nanostructure in nanocomposite films compared to the relevant of pure NZ. This means that the TO@NZ hybrid nanostructure was incorporated significantly better in the poly-

The calculated values of elastic modulus (E), ultimate strength (σuts), and elongation at break (%ε) for all ALG/G/xNZ and ALG/G/xTO@NZ nanocomposite films are listed in

**Figure 9.** (**a**,**c**) SEM images of surface and (**b**,**d**) cross-section for the nanocomposite films of ALG/G/15NZ (**a**,**c**) and ALG/G/15TO@NZ (**c**,**d**) respectively. (**e**) Energy dispersive spectrometer (EDS) spectrum and relative elemental analysis of the surface (inset) from the SEM image (**a**). **Figure 9.** (**a**,**c**) SEM images of surface and (**b**,**d**) cross-section for the nanocomposite films of ALG/G/15NZ (**a**,**c**) and ALG/G/15TO@NZ (**c**,**d**) respectively. (**e**) Energy dispersive spectrometer (EDS) spectrum and relative elemental analysis of the surface (inset) from the SEM image (**a**).

#### It is obvious from Figures 7–9 that after the incorporation into the polymer matrix, *2.6. Tensile Properties*

the increase of the content of NZ or TO@NZ nanocomposite material caused an increase to the aggregation degree. Nevertheless, SEM images of the final nanocomposite films show that the nanohybrids were homogeneously dispersed, which indicates their enhanced compatibility with the polymer matrix. Moreover, SEM surface and cross-section The calculated values of elastic modulus (E), ultimate strength (σuts), and elongation at break (%ε) for all ALG/G/xNZ and ALG/G/xTO@NZ nanocomposite films are listed in Table 1.

images were shown more homogenous dispersion in the case of TO@NZ hybrid nanostructure in nanocomposite films compared to the relevant of pure NZ. This means **Table 1.** Calculated values of Young's (E) Modulus, ultimate tensile strength (σuts) and % strain at break (ε<sup>b</sup> ).


It is obvious from Table 1 that the addition of both NZ and TO@NZ hybrid nanostructure increases stiffness and strength and decreases %elongation at break values. The nanocomposite film with the higher strength was the ALG/G/15NZ and ALG/G/15TO@NZ. This result is in accordance with previous reports where zeolite was successfully incorporated into polyethylene/caprolactone [28], cellulose [29,44], and chitosan [30] films as nano-reinforcement. The result also agrees with the FTIR morphological evaluation of such films where an interplay between NZ, TO@NZ hybrid nanostructures and ALG/G matrix was obtained. In general, ALG/G/TO@NZ based nanocomposite films exhibited higher elongation at break values than the ALG/G/NZ due to the presence of TO molecules which acted as plasticizers [22,45].
